In the building of electronic equipment, it is well known to assemble electronic components onto printed circuit boards which are in turn connected to other portions of the equipment. Conventional printed circuit boards (PCBs) have a dielectric or insulative board upon which traces of a conductive material such as copper are placed, such as by printing, coating and etching or other technique. The traces end at contact points, sometimes called pads or lands, for connection to the various electronic components. Traditionally, the lands or pads, and the insulative substrate beneath them, would be pierced by a hole through which a lead on the electronic component would pass.
In the assembly of such conventional boards, the electronic components would be placed upon the PCB with their leads through the holes in the contact points and soldered in place by conventional means, such as wave-soldering as described in U.S. Pat. Nos. 4,390,120 and 4,360,144, for example. However, in the effort to make electronic equipment and their constituent parts ever smaller, techniques have been devised for reducing the size of the PCB. For example, the holes piercing the lands or pads have been eliminated in favor of surface mounting the components on the PCB. The removal of the holes permits the contact pads to be smaller and allows the traces to be closer together, thereby permitting the size of the PCB to be reduced. The electronic components themselves now have lead shapes, such as gull-wing, butt-leads or J-leads, that allow them to be mounted directly onto the PCB surface. Surface mounting has the additional advantage of permitting components to be mounted on both sides of a PCB having conductive traces on either side.
A disadvantage to surface mounting is that it greatly reduces the contact surface between the component leads and the bonding pad thereby often reducing the physical strength of the bond which may adversely affect the circuit integrity. This reduced contact area also complicates the assembly process by making it more difficult to deliver solder to the bonding site by the conventional techniques. Standard printed wiring boards (PWBs) require wave-soldering for through-hole technology employing mainly "leaded" components. For the surface mounting of so-called "leadless" components, solder paste must be coated or screened on followed by a reflow step. Mixed technology boards, now the most common, require both solder processes. Often more than two soldering cycles are required and more than two types of solder with different melting temperatures are used. And although mounting components on both sides of the PCB potentially doubles the useful space of the PCB, it has proven difficult to mount components on the second side of the board without loosening the components already mounted to the first side. Mounting components on both sides of the board more than doubles the problems involved and steps required.
A number of documents have been published detailing improvements in PCBs. For example, U.S. Pat. No. 2,600,343 teaches a photolithographic technique for patterning conductive traces on a substrate, by providing a tough gelatino-silver-halide stencil on the substrate through which the conductive pattern is electroplated or coated. The pattern may be optionally covered by a protective, cast insulation layer. Similarly, U.S. Pat. No. 2,695,351 uses a lacquer or "stop-off" material as an etch mask to protect insulative areas that are intended to remain. In this patent, the areas of the contact points are not etched at all, whereas the traces between the contact points are partially etched. The partial etching permits the traces to be protected by a separate thermoplastic sheet. This patent contemplates that electronic components would be mounted in through-holes in the PCB.
U.S. Pat. No. 2,848,359 also contemplates a printed circuit using the traditional through-holes for interconnection. Here, a thermoset mask is formed on a substrate having the interconnect holes in place and the exposed portions of the substrate, including the holes, are plated with a conductive material. U.S. Pat. No. 2,874,085 describes a process for forming PCBs by electroplating a circuit through a varnish mask upon a base plate. The base plate is then used as a tool to place the circuit flush in the surface of a plastic sheet, with or without the varnish mask also in place, and the base plate is removed to repeat the process. A disadvantage of this technique is that the pattern must be reversed from that desired since it is flipped and inverted prior to imbedding in the plastic sheet.
Another technique involving the transfer of conductive traces to a substrate is described in U.S. Pat. No. 3,729,819. However, in this case the traces are placed on the surface of the substrate using a base layer of paper or the like, and are not imbedded within the substrate. An adhesive is used to keep the circuit in place. Additionally, the pattern is not reversed in this latter transfer process. U.S. Pat. No. 4,766,268 is directed to a process using at least two layers of lacquer on conventional traces to prevent solder bridging from one land to another. These latter two patents apparently contemplate the surface mounting of components, but do not address the problems raised above with respect to strengthening the bonds and delivering solder to the contact points.
It is noted that in each of the publications referenced above that the circuit traces exist only the x-y plane over the surface of the PCB. These standard PWBs or PCBs have only flat traces on the surfaces. The limitations of this technology are that they use only two dimensions (the x and y dimensions) to accommodate circuit traces and are thus space limited. Most circuits require wide traces for conducting current. Also, the fabrication process is often lengthy and expensive. The process steps can number as high as thirty-six when both through-hole and surface mount technologies are employed. Also, the solderability of traces, pads and through-holes are difficult to maintain during storage because of the very thin layer of protective solder which is applied over the copper. This is particularly true for surface mounted assembly methods.
One technique for increasing the density of traces on the PCB, that is, the number of traces in a given area, or reducing the size of the PCB is to orient the traces vertically or normal to the surface of the board, that is, in the z-direction.
In this manner, the traces may be spaced even closer together. Simply reducing both the vertical and lateral cross-section of the trace would not be advantageous since they would not be able to carry the signal current necessary to assure proper operation of the circuit. Sufficient cross-sectional area must also be provided to minimize the electrical resistance of the trace current path.
Of interest with respect to trench-type, vertical traces is U.S. Pat. No. 4,604,678 which describes a circuit board having traces extending perpendicular to the plane of the surface of the substrate. The grooves are filled by plating with an electrically conductive material. The substrate can be molded from a plateable plastic or ceramic material or the grooves can be machined by a laser beam. However, molded circuit boards with recessed, three dimensional traces are an improvement over flat traces, but the U-shaped traces act as solder thieves in assembly with components and make adequate solder joints extremely difficult if not impossible to achieve.
Although the technique of employing traces extending into the substrate surface has promised much, it remains to be successfully implemented. Such vertical or trench-like traces aggravate the problems of bonding processes, including the great plurality of steps, poor bonding strength and difficult solder delivery mentioned earlier. Additionally, while it is relatively easy to envision a circuit board with vertically-oriented traces, it is considerably less apparent how such traces are to be formed.
Another circuit board assembly that has been envisioned, but not easily produced is one where integrated circuits or "chips" are directly placed on the circuit board without the need for encasing them in a separate package, such as a plastic or ceramic. Tape automated bonding (TAB) of a small, fragile lead frame to a chip also holds great promise, but traditionally has been a difficult technology because the TAB leads are easily damaged before mounting the device on a circuit board or other substrate.